The hypothesis to be tested is that astrocytes and microglia express the co stimulatory molecules, B7 1 and B7 2, in a differential manner and that the expression of these molecules affects the direction of the immune response made by myelin reactive T cells with which the glial cells interact. Experimental autoimmune encephalomyelitis (EAE) is a demyelinating disease of the central nervous system (CNS) mediated by activated CD4+ T cells, which serves as a model for multiple sclerosis (MS). T cell activation involves antigen nonspecific interactions termed "co stimulation". The co stimulatory molecules B7 1 and B7 2 are ligands for the CD28 and CTLA 4 receptors on T cells. principal investigator laboratory and others have shown that CD28 mediated T cell signaling is crucial to the development of EAE. CD28 mediated signaling results in T cell proliferation and cytokine production. For disorders such as MS and EAE, in which activation of T cells in the target CNS is believed to be necessary, it is important to identify cells within the target organ capable of activating T cells and the factors which regulate this ability. Expression of the other B7 receptor, CTLA 4, is confined to activated T cells. Signaling through CTLA 4 leads to T cell inhibition and blockade of the CTLA 4/B7 interaction worsens EAE. Thus, B7 1 and B7 2 expressing cells within the CNS may regulate activated T cells expressing CTLA 4. principal investigator has preliminary data that B7 1 mRNA and immunoreactivity can be induced in cultured astrocytes, and that B7 1 is expressed by astrocytes in EAE affected CNS. Aim 1 will examine the induction and differential regulation of B7 1 and B7 2 in highly purified cultured astrocytes and microglia. Aim 1 will also examine effects on expression of selected immune related molecules by astrocytes or microglia upon the ligation of B7 1 and B7 2 molecules expressed by the glia. Aim 2 will analyze the cellular expression of B7 1 and B7 2 in tissues and cells derived from EAE affected animals, which will also allow us to identify cells other than astrocytes or microglia that might express B7 1 or B7 2. Aim 3 will address the functions of B7 molecule expression by astrocytes and microglia, by determining whether expression of these molecules can activate T cells to proliferate, produce cytokines and transfer EAE to naive recipients, or can inhibit T cells or lead to T cell apoptosis. The principal investigator proposes studies to identify resident CNS cells expressing B7 1 and/or B7 2, their role in T cell regulation, and the effect of B7 ligation upon the glial cell itself, and how this may enhance the understanding of the pathogenesis of EAE and MS, and perhaps suggest areas for therapeutic development.